Analysis of Nitrogen Removal Processes in a Subsurface Flow Carbonate Sand Filter Treating Municipal Wastewater

Abstract

Controlled experiments were carried out in a mesoscale subsurface flow sand filter treating municipal wastewater from a single household. The system consisted of a 50 cm high vertical flow column (pre-filter) with unsaturated flow and a 3 m long horizontal sub-surface flow unit (main filter) with saturated flow. Fluxes of nitrogen and carbon were analyzed in 4 different operating conditions (low and high loading, with and without the prefilter unit). Water samples were taken from the inlet, the outlet and within the sand filter at different depths and locations and analysed for water quality (Tot N, NO₃-N, NH₄-N, TOC, DOC, CODcr, BOD₅, SS, pH, and EC) and dissolved gas content (N₂O, CH₄, and CO₂). Emissions of N₂O, CH₄, and CO₂ were measured with the closed-chamber technique adjacent to water quality sampling points. The results show that prefiltering in a vertical, unsaturated flow column changed the incoming ammonium to nitrate during low loading. During high loading part of the ammonium nitrified in the pre-filter was lost by denitrification. Within the horizontal main filter there were two pathways for the incoming nitrate: denitrification and dissimilatory nitrate reduction to ammonium (DNRA).

Key Words:

• DNRA
• Denitrification
• Nitrification
• Sand filter
• Carbon.

Notes

^a Iron extracted with ammonium-oxalate.

^b Extraced with aqua regia.

f = filtererd with GF/C filter.

*N₂ emission estimated from N mass balance.